That is a mix of the sebum already on your head and too much baking soda. I had the same problem. Although I washed for the past year with nothing but baking soda, once I dreaded my head I noticed that nothing was actually getting anything out. Baking soda is my go to for everything that I clean in my house, but I opted against using it on my head because no matter what I did I couldn't get the right portions and it wasn't getting my scalp clean. It's a pretty weird feeling. I don't want to sound like a commercial but I mix between using dr. bronners or anything soap that doesn't sud up every other wash. I notice that after using apple cider my dreads are left greasy. And baking soda works better if you literally soak your head in a bucket rather than pouring it over your head in a shower. That's just so I can ensure to get everything out. I know that your sebum will go crazy at first as it is trying to balance it out.